1. Field of the Invention
The present invention relates to an optical fiber coating method and coating apparatus; and, in particular, to those adapted to stably apply a coating resin in a higher linear velocity region.
2. Related Background Art
In the making of optical fibers, there has conventionally been employed a method comprising the steps of drawing an optical fiber by pulling an optical fiber preform while heating and melting it; applying two different types of resins to the surface thereof by use of a die adapted to apply two layers at the same time; curing the resins by use of a curing apparatus; and taking up the optical fiber with a take-up apparatus by way of a capstan, a pulley, and the like. An apparatus which applies two different types of resins by use of a die adapted to apply two layers at the same time is disclosed in Japanese Utility Model Application Laid-Open No. HEI 2-38437, Japanese Patent Application Laid-Open No. HEI 9-86971, or the like.
FIG. 16 is a sectional view of the conventional optical fiber coating apparatus disclosed in Japanese Patent Application Laid-Open No. HEI 9-86971. An optical fiber 1 comes into contact with a first coating resin A when passed through a die hole 30 of a first coating die 3 from a nipple hole 20. The optical fiber 1 covered with the first coating resin A further comes into contact with a second coating resin B when passed through a die hole 40 of a second coating die 4 from the die hole 30, and then is drawn out of the die hole 40, whereby the optical fiber 1 with a double coating is fabricated.
From a first coating resin introduction hole 71 provided in the upper part of an outer sleeve 7, the first coating resin A is successively supplied to an outer peripheral groove 65 of an inner upper sleeve 6a, a hole thereof, a first reservoir 11 disposed at the outer periphery of the nipple 2, a first draw portion 9, a first coating resin orthogonal flow path 10 formed by the tip surface of the nipple 2 and the upper face of the first coating die 3, and a portion between the outlet of the nipple hole 20 and the inlet of the die hole 30. From a second coating resin introduction hole 72 provided in the lower part of the outer sleeve 7, the second coating resin B is successively supplied to an outer peripheral groove 66 of an inner lower sleeve 6b, a hole thereof, a second reservoir 12 disposed at the outer periphery of the second coating die 4, a second draw portion 13, a second coating resin orthogonal flow path 14 formed by the lower face of the first coating die 3 and the upper face of the second coating die 4, and a portion between the outlet of the die hole 30 and the inlet of the die hole 40.
It is described that the first coating resin 11 is thus once stored in the first reservoir 11 and then is sufficiently narrowed by the first draw portion 9, thereby being regulated so as to form a uniform flow throughout the periphery, by which the optical fiber 1 is provided with a coating having a uniform thickness. On the other hand, the first coating resin orthogonal path 10 is formed between the outlet end face of the nipple hole 20 and the inlet end face of the die hole 30 and intersects the optical fiber 1 at right angles. It is also described that the first coating resin A consequently flows orthogonal to the optical fiber 1, and thus can stably be applied to the latter while suppressing the recirculation thereof.
However, such a conventional coating apparatus has a drawback in that the outside diameter of the coating fluctuates as the drawing speed increases.
The inventors studied causes of the outside diameter fluctuation in detail by simulations and experiments. As a result, it has been found that, if the outlet portion of the die hole 30 at the lower face of the first coating die 3 is flat, then an unstable annular lower-pressure region is formed near this outlet, whereby the first coating resin A applied to the outer periphery of the optical fiber 1 is pulled by the annular lower-pressure region, so as to irregularly expand to the outside, thus forming the outside diameter fluctuation in the coating.
For eliminating the outside diameter fluctuation of the coating, the inventors have found it effective to provide a protrusion for regulating flows near the outlet. The present invention is based on this finding.
Here, the results of CFD (Computational Fluid Dynamics) simulations will be explained in brief.
Flow analysis program CFX 4.1 (Flow Solver) manufactured by AEA Technology PLC was used for the simulations, and influences of the form of the outlet portion were mainly studied. The analysis condition is shown in the following Table.
Under this analysis condition, two kinds of cases, i.e., case 1 in which the first layer die hole outlet is flat (corresponding to the conventional coating apparatus) and case 2 in which the first layer die hole outlet is provided with a beak-shaped protrusion having a height of 0.1 mm and a skirt width of 0.1 mm (corresponding to the coating apparatus of the present invention), were analyzed. FIGS. 1 and 3 show the respective pressure distributions near the first layer die hole outlet of the two cases, whereas FIGS. 2 and 4 show the respective stream line vector distributions near the first layer die hole of the two cases.
In case 1 corresponding to the conventional coating apparatus, as shown in FIG. 1, it can be seen that an annular lower-pressure region widely spreads near the first layer die hole outlet. Consequently, the resin coming out of the first layer die hole is outwardly pulled in the vicinity of the outlet as shown in FIG. 2. As a result, a bulge occurs in the outside diameter of the coating.
In case 2 corresponding to the coating apparatus of the present invention, on the other hand, as shown in FIG. 3, it can be seen that the annular lower-pressure region becomes smaller. Consequently, resin flows substantially align in parallel with the optical fiber drawing direction as shown in FIG. 4, whereby the outside diameter of the coating becomes more stable than that in case 1.
The inventors also studied influences of the form of the protrusion provided near the outlet by CFD simulations. FIGS. 5A to 5D show their results, and are views schematically showing lower-pressure regions generated near the outlet of the first die hole 30 and their resulting outside diameter fluctuations. In the case shown in FIG. 5A provided with no protruded member, a lower-pressure region 15 of second coating resin B near the optical fiber 1 spreads over a wide area, whereby a bulge 11 of first coating resin A becomes remarkable. In the case with a protrusion shaped like a circular truncated cone as shown in FIG. 5B, on the other hand, it has been found that the lower-pressure region 15 near the optical fiber 1 is suppressed, so as to substantially eliminate the bulge 11 of first coating resin A. It has also been found that, in the case where the protruded member is substantially conical or shaped like a flat circular truncated cone with a large head or bottom portion as shown in FIG. 5C or 5D, the effect on reducing the dimensions of the lower-pressure region 15 tends to be smaller.
Also, the inventors compared these CFD simulation results with experimental results, and have confirmed the effect on suppressing the outside diameter fluctuation obtained when the protrusion is shaped like a circular truncated cone with a small head portion.
The present invention is based on these findings, and it is an object of the present invention to provide an optical fiber coating method and coating apparatus which, when the optical fiber is covered with a resin applied thereto, can improve the applicability of the resin to the optical fiber, and by which the coating resin can stably be applied to the optical fiber without outside diameter fluctuation, in particular, in a higher linear velocity region of the optical fiber.
The optical fiber coating method in accordance with the present invention comprises the steps of: (1) applying a first coating resin to the outer periphery of an optical fiber by injecting a first coating resin into a clearance between a first die hole and the optical fiber, while inserting the optical fiber through the first die hole provided in a first coating die and having an inside diameter greater than an outside diameter of the optical fiber; and (2) applying a second coating resin onto the first coating resin by injecting a second coating resin into a clearance between a second die hole and the surface of the first coating resin applied to the optical fiber, while inserting the optical fiber through the second die hole provided in the second coating die and having an inside diameter greater than that of the first die hole. Wherein a disk-shaped upper end face of the second coating die and a basically disk-shaped lower end face of the first coating die having a protrusion formed around the first die hole and projecting in the passing direction of the optical fiber are opposed to each other so as to arrange the first and second die holes concentrically, and the second coating resin is injected into the second die hole by way of a gap formed between the lower end face of the first coating die and the upper end face of the second coating die, so as to reduce an annular lower-pressure region formed around the optical fiber in a flow of the second coating resin within the gap.
And the optical fiber coating apparatus in accordance with the present invention is an optical fiber coating apparatus for applying first and second coating resins as a laminate to the outer periphery of an optical fiber, the apparatus comprising a first coating die and a second coating die; the first coating die having a first die hole through which the optical fiber is inserted and a basically disk-shaped lower end face with a protrusion projecting in the passing direction of the optical fiber and formed around the first die hole, the first die hole and the outer periphery of the optical fiber therein forming a space therebetween into which the first coating resin is injected; the second coating die having a second die hole which is concentric with the first die hole and through which the optical fiber passed through the first die hole is inserted and an upper end face comprising a circular plate opposing the lower end face of the first coating die so as to form a gap through which the second coating resin is injected into a space formed between the second die hole and the outer periphery of the optical fiber therein. The protrusion being formed so as to reduce an annular lower-pressure region formed around the optical fiber in a flow of the second coating resin within the gap.
According to the present invention, the first coating resin is applied to the outer periphery of the optical fiber through the first die hole of the first coating die. Also, the second coating resin is supplied from the surroundings of the gap formed by the first and second coating dies into the second die hole at the center part, so as to be applied onto the first coating resin. Here, since the first die hole outlet is formed to have a protrusion which is shaped so as to project toward the exit, i.e., toward the second die hole, and reduce an annular lower-pressure region within the second coating resin which flows as mentioned above, the flow of second coating resin is regulated so as to align with the flowing direction of the optical fiber. As a consequence, the inserted optical fiber is restrained from vibrating, whereby the first and second coating resins can uniformly be applied to the optical fiber at the first and second die holes, respectively. Also, since the tip of the protrusion approaches the inlet of the second die hole, it is effective in the prevention of vibration.
As a result of the above-mentioned studies, the inventors have found it preferable for the protrusion to be shaped like a circular truncated cone and, in particular, to satisfy the following relationships:
0.05G less than H less than 0.5G 
(D2xe2x88x92D1)/2 less than W less than G 
0.01 mmxe2x89xa6L less than W
where H is the height of the circular truncated cone of the protrusion, W is the distance between the outer periphery of the bottom portion of the circular truncated cone and the inner peripheral face of the first die hole, L is the distance between the outer periphery of the head portion of the circular truncated cone and the inner peripheral face of the first die hole, D1 is the inner peripheral face diameter of the first die hole on the outlet side of the optical fiber, D2 is the inner peripheral face diameter of the second die hole on the inlet side of the optical fiber, and G is the distance of the gap between the first and second coating dies.
Preferably, the optical fiber coating apparatus in accordance with the present invention further comprises a positioning member having a cylindrical inner peripheral face adapted to fit the respective outer peripheral faces of the first and second coating dies, each of the first and second coating dies and the inner peripheral face of the positioning member being constituted by a material having a Young""s modulus of 5xc3x97104 kg/mm2 or greater and a coefficient of thermal expansion of 6xc3x9710xe2x88x926/xc2x0 C. or lower.
In this configuration, the inner peripheral face of the positioning member and each of the coating dies fitted therein are constituted by a material having a high hardness suitable for precision processing. Therefore, when the first and second coating dies are fitted into the positioning member, the fitting clearance between the positioning member and each coating die can be made smaller. As a consequence, the alignment between the center axis of the hole of the first coating die and the center axis of the hole of the second coating die can easily be carried out with a high accuracy. Further, since the coating dies and at least the inner peripheral face of the positioning member are constituted by a material having a lower coefficient of thermal expansion, deformations due to thermal expansion can be suppressed even if the temperature of members rises while the resin is being applied to the optical fiber. Hence, the first coating die hole and the second coating die hole can be restrained from positionally shifting from each other, whereby a coating with less eccentricity can be applied to the optical fiber.
Preferably, this positioning member is constituted by an inner periphery member made of cemented carbide forming the inner peripheral face and an outer periphery member made of alloy tool steel having a lower Young""s modulus and a higher coefficient of thermal expansion than the inner periphery member which are fastened and secured together by interference fitting.
If the whole positioning member is constituted by cemented carbide, then it becomes hard to process into a complicated form, though the fitting clearance between the positioning member and the coating dies can be made smaller. If the whole positioning member is constituted by alloy tool steel having a hardness lower than that of cemented carbide, on the other hand, then it becomes hard to maintain the accuracy of processing, though the member can easily be processed into a complicated form, whereby the fitting clearance increases so much that highly accurate alignment cannot be expected. When the positioning member has a double structure in which the inner periphery portion is made of cemented carbide, whereas the outer periphery portion is made of alloy tool steel, the fitting gap between the positioning member and each coating die can be kept smaller as mentioned above. As a consequence, the misalignment between the respective center axes of both die holes can be reduced. Further, since deformations of the coating apparatus upon thermal expansion are suppressed even if temperature rises, a coating with less eccentricity can be applied to the optical fiber. Also, since the outer side of the positioning member is constituted by alloy tool steel, this portion can easily be provided with a coating resin flow path having a complicated form and the like.
Preferably, the bottom face of the first or second die has a tap for use upon attachment/detachment with respect to the positioning member.
In the optical fiber coating apparatus in accordance with the present invention, the first and second coating dies are secured within the positioning member by fitting. When the bottom face of the first or second coating dies is provided with a tap, the attachment/detachment between the first or second coating die and the positioning member can be carried out easily and reliably for the cleaning of the inside of the coating apparatus and the like after coating. As a consequence, workability improves, and damages and deterioration in positioning accuracy can be prevented from occurring upon attachment/detachment.
Preferably, there is further provided a nipple made of a material having a Young""s modulus, a coefficient of thermal expansion, and a hardness which are substantially identical to those of the inner peripheral face of the positioning member, the nipple being adapted to fit the inner peripheral face of the positioning member such that a nipple hole for guiding the inserted optical fiber to the first die hole is arranged concentric with the first die hole. When such a nipple is provided, the misalignment between the nipple hole and both die holes can be reduced, whereby a coating having a less eccentricity can be formed around the optical fiber.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.